1. I use a computer everyday
I use computer programs to prepare teaching materials.
2. I play the piano everyday
I have another job - I play piano in a band every weekend.
in this instance you can use either �piano� or �the piano�, as in every case I can think of while writing this post...
3. play the �cello or play �cello ( �cello is short for violincello) - this is the same as for 2.
I play �cello in a youth orchestra but violin at school
I play the �cello because I love the way it sounds.
For 2 & 3 �the � would generally inmply a specific instrument whereas the bare instrument name implies a musical instrument of that type/family.
Cheers,
AB